Branching (P2)¶

This page introduces branching: how a Python program makes decisions and chooses different paths of execution.

Branching depends directly on operators. If operators produce True or False, branching decides what to do next.

What Is Branching?¶

Branching is the ability of a program to execute different code blocks depending on a condition.

In Python, branching uses:

• if
• elif
• else

Example:

if score >= 6.0:
    result = "high"

If the condition is True, the block runs. If it is False, the block is skipped.

The if Statement¶

An if statement evaluates a condition.

if value > 0:
    print("Positive value")

• The condition must evaluate to True or False
• The indented block runs only when the condition is True

if / else¶

Use else when there are exactly two paths.

if count == 0:
    print("No records")
else:
    print("Records found")

Only one branch will run.

if / elif / else¶

Use elif (else if) when there are multiple possible conditions.

if score >= 7.5:
    level = "high"
elif score >= 5.0:
    level = "medium"
else:
    level = "low"

Rules:

• Conditions are checked top to bottom
• The first True condition wins
• Only one block executes

Branching with Boolean Operators¶

Conditions often combine multiple comparisons using Boolean operators.

if score >= 4.0 and score <= 7.5:
    status = "typical"

Branching with Membership Tests¶

Membership operators are common in data work.

if file_type in ["csv", "json", "excel"]:
    process_file()

Branching for Data Validation¶

Branching is often used to skip invalid data.

if value is None:
    return

or:

if value < 0:
    logger.warning("Invalid value")

Indentation Matters (Python Rule)¶

Branching blocks are defined by indentation, not braces.

if condition:
    do_this()
    do_that()

Common Mistakes¶

Forgetting the colon¶

# WRONG
if score > 5

# RIGHT
if score > 5:

Misaligned indentation¶

if score > 5:
print("High")  # WRONG

How Branching Fits into Pipelines¶

Branching is used to:

• validate inputs
• skip bad rows
• choose output paths
• handle edge cases

Reminders¶

• Branching controls which code runs
• Conditions rely on operators
• Only one branch runs per if chain
• Branching is essential for real data work